The present invention relates generally to an exposure apparatus and a method used to fabricate various devices, including semiconductor chips such as ICs and LSIs, display devices such as liquid crystal panels, sensing devices such as magnetic heads, and image pickup devices such as CCDs, as well as fine patterns used for micromechanics, and, more particularly, to an immersion type exposure method and apparatus for immersing the final surface of the projection optical system and the surface of the object in the fluid and exposing the object through the fluid.
The projection exposure apparatus employed to manufacture a semiconductor device, a liquid crystal display device, etc., is required to use an increased numerical aperture (“NA”) of a projection optical system and a shortened wavelength of an exposure light. A KrF stepper (with λ=about 248 nm) used an NA of 0.65 in 2000. The projection exposure apparatus is being shifted from the stepper to a scanner to which a high NA projection lens and KrF and ArF (λ=about 193 nm) lasers are applicable. The current scanners use a projection lens having an NA up to 0.8 with a light source, such as KrF and ArF lasers. A prospective scanner that will appear in the market within several years will use a projection optical system having an NA of 0.85 and a light source of KrF, ArF, and even F2 (λ=about 157 nm) lasers. Discussions have started about whether it is possible to develop a projection optical system having an NA up to 0.95 of a dry system that uses the gas for a medium between lenses as an ultimate projection optical system for the photolithography.
An antireflection coating for an optical element progresses as the projection optical system progresses. When the projection exposure apparatus appeared with a light source of KrF and ArF excimer lasers in 1990, the coating material for the antireflection coating became restricted, such as SiO2 and MgF2 as a low refractive index material having a refractive index between 1.45 and 1.55, and Al2O3 and LaF3 as a middle refractive index material having a refractive index between 1.65 and 1.75. This limit increases the design difficulty and requires a reduction of the transmission loss due to absorptions in the coating, contaminated substrate, and scattering in the coating layer, which has conventionally been negligible.
In this background, the immersion exposure has attracted attention as one measure to improve the resolution while using the ArF and F2 lasers (see, for example, Japanese Patent Application, Publication No. 10-303114). The immersion exposure promotes the higher numerical aperture (“NA”) by replacing a medium (typically, air) at the wafer side of the projection exposure with the fluid. The projection exposure apparatus has an NA of n·sin θ, where n is a refractive index of the medium, and the NA increases when the medium that has a refractive index higher than the refractive index of air, i.e., n>1. For example, when the fluid is pure water (with a refractive index of n=1.33), and the maximum incident angle of the light that images on the wafer is the same between the dry system and the wet system, the wet system's resolving power improves by 1.33 times. In other words, the NA in the wet system corresponds to 1.33 times as large as the NA of the dry system. If the apparatus's limit of the incident angle of the light that images on the wafer is 70°, the NA of the dry system has a limit of 0.94, whereas the NA of the wet system has a limit of 1.25 (when converted into the dry system), providing a projection optical system with a high NA. When the final surface of the projection lens is contaminated in the immersion projection optical system, the immersed optical element should be easily replaced. Therefore, Japanese Patent Application, Publication No. 10-303114, proposes a structure that provides the final surface of the projection lens with a plane-parallel plate.
As described by this inventor in Japanese Patent Application No. 2003-135578, the P-polarized light reflectance of the antireflection coating in the high NA projection optical system remarkably increases, as shown in FIG. 7, when the antireflection coating receives the light from the air layer and exceeds the Brewster angle that is determined by the refractive index of the final layer of the antireflection coating (at the air side). In general, it is usual to use a low refractive index material for the antireflection coating's final layer that contacts the air so as to design the reflectance to be reduced in the wide incident-angle range. Even when the basic coating design changes, as shown in FIG. 8, the reflectance becomes similar values, as the incident angle increases, if the material of the final layer that contacts the air is the same. Although the phase change between the P-polarized light and the S-polarized light is negligible in a range below the Brewster angle, their transmission phase changes become large once the angle exceeds the Brewster angle and the influence to the aberration of the projection optical system cannot be negligible.
Therefore, the antireflection coating having a limited usable coating material for the F2, ArF and KrF lasers disadvantageously deteriorates both the transmittance and the imaging performance of the projection optical system once the NA of 0.85 or 58° of the light exceeds the Brewster angle. It is also necessary to consider the limits of the antireflection coating in the projection optical system of the wet system. While the immersion exposure apparatus disclosed in Japanese Patent Application, Publication No. 10-303114, uses the plane-parallel plate for the final optical element in the projection optical system and facilitates the replacement, an immersion structure of the plane-parallel plate at only the wafer side poses a problem of the total reflection at the interface between the plane-parallel plate and the air layer at the side of the projection optical system when the NA of the projection optical system exceeds 1.0.